Subfloor requirements for wood flooring

At a glance

Figures are those published in two UK flooring suppliers' installation guides (January and May 2024) and our own guide. BS 8201:2011 was not read: confirm against the current edition and the floor maker's instructions, which prevail.

The building comes first

The floor should be one of the last things to go in. UK installation guidance asks for the building to be weathertight, with doors and windows fitted, and for all wet trades to be complete and dried out before the flooring is delivered. One distributor's guidelines (January 2024) ask for 45 to 65% relative humidity and 15 to 25 °C during installation, and for those conditions to be kept for the life of the floor. Our own installation guide asks for 16 to 24 °C in the same humidity band, and one manufacturer's guide gives an optimum of 40 to 60% RH and 18 to 22 °C. The ranges overlap but are not identical, so state the band you want, and use the floor maker's own figures if they are tighter.

Monitor with a thermometer and hygrometer from delivery to completion, and keep the log. Delivery, storage and acclimatisation are covered in timber floor acclimatisation and site conditions.

Moisture: the numbers differ, so write your own down

Moisture is the check that most often goes wrong, and the published numbers do not agree. The table sets out what four documents say. They differ because they use different tests (relative humidity by hygrometer, or the carbide CM method), different floors (bonded or floated, heated or not) and different assumptions about a damp-proof membrane. None of them is wrong for its own floor. What matters is to use the limit the floor maker names for the product, write it into the specification with the test method, and require the reading.

Figures are as written in each document, with the supplier's own units. BS 8201:2011 was not read, and no figure here is attributed to it.

Four points hold across the sources. The test for a cement or calcium sulphate base is a hygrometer test, which BS 8201:2011 sets out in Annex A (titled ‘Hygrometer test for dampness of concrete, cementitious and anhydrate bases’). A wood moisture meter is not for concrete: it reads differently and needs calibrating to the species. A calcium sulphate screed has its laitance, the weak surface layer, sanded off and the surface primed before the floor goes down, and one distributor says liquid moisture suppressants cannot be used on it. And a reading is a snapshot: it tells you whether to start, not what the screed will do later.

Where a reading is above the limit, the options are to wait, to dry the building, or to apply a surface damp-proof membrane that the floor maker accepts for that floor and that screed.

Flatness: SR1 and the straight edge

Flatness is a different check from level. Level says whether the floor is at the right height across a room; flatness says whether the surface is smooth over a short distance, and it is flatness that opens gaps in a pattern floor. Both floor makers' guides we read ask for the subfloor to be flat to ‘British Standard SR1 tolerance’: a maximum gap of 3 mm under a 2 m straight edge, at any point across the subfloor. SR1 is the class name; the 3 mm and the 2 m are the figures the makers quote. We could not confirm which part of the BS 8204 series defines the class, so check that with the screed specifier.

Check how the number is measured, because it is not always the same test. One supplier's example specification, written in the NBS style, asks for a 3 m straight edge with 3 mm feet at each end and no gap greater than 6 mm. That is a different test, and its figure is not comparable with 3 mm under 2 m. Name the straight edge, the method and the limit in your specification.

Flatness over 2 m says nothing about level across a room, or about falls. If level matters, for example where the new floor must meet an existing one, specify a level tolerance and a level survey separately. One manufacturer notes that a poured or pumped screed rarely reaches SR1 on its own and recommends a self-levelling compound under a timber floor. Pattern floors are the least forgiving: see the parquet pattern atlas for what herringbone and chevron ask of the base.

Screeds, heating and overlays

• Sand-cement and calcium sulphate screeds must be sound, with no friable areas, and free from laitance, dust and cracks. A calcium sulphate screed needs its laitance sanded off and a suitable primer applied before the adhesive goes down.
• Bond strength. One manufacturer asks for a tensile strength of at least 1.5 MPa for a fully bonded floor. Do not leave this until the floor arrives: ask the screeder for evidence that the screed reaches it.
• Underfloor heating. The system should be commissioned and run before the floor goes down, so that moisture deep in the screed is driven out. One distributor asks for a maximum floor temperature of 27 °C and for the system to be left running for two weeks before installation. One manufacturer asks for it to be brought up to temperature over several days, run for five days, then cooled, and for the screed moisture to be recorded afterwards. See wood flooring over underfloor heating.
• Timber and plywood bases. Sound, tested with a moisture meter, with a ventilated void under a suspended floor and no decay or insect attack. One distributor asks for plywood of at least 6 mm (BS EN 314-1 Class 3) laid at 90 degrees to the existing boards, and at least 9 mm glued and fixed under parquet. One manufacturer asks for vertical movement of less than 5 mm.

Who hands over what

Most disputes about a failed floor turn on who was responsible for the subfloor. Say it in the employer's requirements or the specification before the floor is ordered. The allocation below is typical, not a rule from a standard: set it in your own contract with your quantity surveyor or contract administrator.

What the floor contractor should receive before the floor is delivered:

• The subfloor type, the screed product and thickness, and the date it was laid.
• The hygrometer readings: date, relative humidity, temperature, meter type and a plan of the test points.
• The flatness survey: the method, the class and anything that was corrected.
• The underfloor heating commissioning record, if there is one: dates, maximum temperature and the last day of running.
• Details and date of any primer or damp-proof membrane.
• The site temperature and humidity log.
• The floor contractor's written acceptance of the subfloor.

Screeds dry at different rates with thickness, product, ventilation and weather, and a screed that feels dry can still be too wet. Programme the floor from a reading, not from a calendar date.

Subfloor flatness [SR1]: no gap greater than [3] mm under a [2] m straight edge placed anywhere on the surface. Method: [2 m straight edge resting directly on the surface; gap read with a wedge or feeler gauge]. Areas outside the limit to be corrected by [the screeding contractor] before the floor contractor starts. Level survey to [ ] mm over [ ] m to be issued with the flatness results.

Relative humidity not to exceed [65]% [75% where a surface damp-proof membrane is specified] when tested with a hygrometer to BS 8201:2011 Annex A, or the limit in the floor manufacturer's instructions if lower. Test points at corners, around the perimeter and at random across the area, [ ] points in all. Readings to be recorded (date, RH, temperature, meter type, plan of test points) and issued to the floor contractor [before the flooring is delivered].

Timber or wood-based subfloor sound, ventilated and free from decay and infestation, moisture content not exceeding [14]% WME and within [2]% of the flooring, or the floor manufacturer's limit if lower. Where specified, [ ] mm plywood overlay to BS EN 314-1 Class 3, [fixed at 90 degrees to the existing boards].

No flooring to be laid until the floor contractor has inspected the subfloor and confirmed in writing that it meets the limits above. Underfloor heating to be commissioned and its record issued before delivery. Corrective work: [screeding contractor / main contractor].

Standards and sources

• BS 8201:2011: code of practice for installation of flooring of wood and wood-based panels; Annex A is the hygrometer test for concrete, cementitious and anhydrate bases (title as quoted by suppliers). Paywalled: not read; confirm against the current edition.
• BS 8204 series: screeds, bases and in situ floorings. Floor makers cite an ‘SR1’ class; which part defines it was not confirmed. Not read.
• BS EN 13813 and BS EN 314-1: screed materials; plywood bonding quality (Class 3 overlay plywood, as cited by one distributor). Named; not read.
• UK flooring supplier documents read 5 Nov 2026: a distributor's installation guidelines (January 2024); a manufacturer's bonded-floors guide (7 May 2024); an NBS-style example specification (March 2020); our own installation guide (12 October 2026).

Questions we are asked

What relative humidity must a screed be before laying wood flooring?

Published UK supplier guidance gives under 65% RH by hygrometer for a cement or calcium sulphate screed, rising to under 75% where a surface damp-proof membrane or vapour check is applied; one specification template gives 75% as a maximum, and some guides use the carbide (CM) method instead. The numbers differ between floors, so use the limit the floor maker names and the test in BS 8201:2011 Annex A, which this sheet has not read.

What does SR1 mean?

SR1 is a surface-regularity class that UK floor makers quote as a maximum gap of 3 mm under a 2 m straight edge, at any point across the subfloor. We could not confirm which part of BS 8204 defines it, so check the class and the limit with your screed specifier.

How do you check subfloor flatness?

Lay a straight edge on the surface and measure the largest gap beneath it with a wedge or feeler gauge, at several places and in different directions. Floor makers quote a 2 m straight edge resting directly on the surface; one specification template uses a 3 m straight edge on 3 mm feet, which is a different test. Say which one you mean.

Who is responsible for a subfloor that is out of tolerance?

In practice the builder or screeding trade, who hand over the base, and our installation guide says it is usually the builder's responsibility to hand the floor contractor a subfloor within tolerance. The floor contractor should inspect it and accept it in writing. Put the allocation in the specification or employer's requirements before the floor is ordered.

Can wood flooring go over underfloor heating?

Yes, with preparation. Suppliers ask for the system to be commissioned, brought up to temperature and run before the floor goes down, with a maximum floor temperature, and for the screed moisture to be read afterwards. The limits are tighter for calcium sulphate screeds with heating. See our guide to wood flooring over underfloor heating.

Is a moisture meter reading enough for a screed?

No. A wood moisture meter is not for concrete or screed: it reads differently and needs calibrating to the species. The test for a cement or calcium sulphate base is the hygrometer test in BS 8201:2011 Annex A, or the carbide method where the floor maker names it.